Single-focus lens

ABSTRACT

A compact, inexpensive single-focus lens in a three-element configuration comprises, successively from an object side, a first lens with a low power having at least one aspheric surface, a second lens with a positive refracting power having a convex form on an image surface side, and a third lens with a low power having at least one aspheric surface.

RELATED APPLICATIONS

[0001] This application claims the priority of Japanese PatentApplication No. 2001-156653 filed on May 25, 2001, which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a compact single-focus lensconstituted by three elements and, in particular, to a single-focus lenssuitable for digital cameras.

[0004] 2. Description of the Prior Art

[0005] Along with the widespread use of personal computers, digitalcameras which can easily process captured image information have beenbecoming pervasive in recent years.

[0006] Such digital cameras are required to be more compact and lessexpensive, which causes an urgent demand for their taking lenses toachieve compactness and low cost.

[0007] As taking lenses for fulfilling such a demand, those disclosed inJapanese Unexamined Patent Publication No. HEI 9-258100, JapaneseUnexamined Patent Publication No. 2000-180719, and the like have beenknown.

[0008] However, each of the taking lenses disclosed in theabove-mentioned publications uses at least four lenses, and thus isdemanded to achieve further compactness and lower cost.

[0009] Meanwhile, digital cameras use small-sized electric imagingdevices, thereby making it necessary for principal rays to be madeincident on the whole screen substantially at right angles, whereas ahigher aperture efficiency and a longer back focus are required, and soforth, whereby the idea of designing their taking lens is greatlydifferent in principle from that in conventional compact cameras usingsilver halide films.

SUMMARY OF THE INVENTION

[0010] In view of circumstances mentioned above, it is an object of thepresent invention to provide, as a taking lens mounted to a compactcamera, a digital camera in particular, a single-focus lens which canachieve compactness and lower cost while favorably correctingaberrations by using a three-lens configuration.

[0011] The single-focus lens in accordance with the present inventioncomprises, successively from an object side, a first lens with a lowpower having at least one aspheric surface, a second lens with apositive refracting power having a convex form on an image surface side,and a third lens with a low power having at least one aspheric surface.

[0012] Preferably, a stop is disposed between the first and secondlenses.

[0013] Preferably, the first and second lenses are formed from a plasticmaterial.

[0014] Preferably, the single-focus lens of the present inventionsatisfies the following conditional expressions (1) to (3):

|f′/f ₁′|<0.5  (1)

0.5<f′/f ₂′<2.0  (2)

|f′/f ₃<0.5  (3)

[0015] where

[0016] f′ is the focal length of the whole lens system;

[0017] f₁′ is the focal length of the first lens;

[0018] f₂′ is the focal length of the second lens; and

[0019] f₃′ is the focal length of the third lens.

[0020] Preferably, the first lens has a meniscus form with a concavesurface directed onto the object side.

[0021] Preferably, the single-focus lens of the present inventionfurther satisfies the following conditional expressions (4) and (5):

1.65>N_(d2)  (4)

50<ν_(d2)  (5)

[0022] where

[0023] N_(d2) is the refractive index of the second lens at d-line; and

[0024] ν_(d2) is the Abbe number of the second lens at d-line.

[0025] The second lens may have a substantially flat surface on theobject side.

[0026] Each of the first and third lenses may have an aspheric surfaceon each side thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic view showing the configuration of thesingle-focus lens in accordance with Example 1 of the present invention;

[0028]FIG. 2 is a schematic view showing the configuration of thesingle-focus lens in accordance with Example 2 of the present invention;

[0029]FIGS. 3A to 3C are aberration charts showing various aberrations(spherical aberration, astigmatism, and distortion) of the single-focuslens in accordance with Example 1; and

[0030]FIGS. 4A to 4C are aberration charts showing various aberrations(spherical aberration, astigmatism, and distortion) of the single-focuslens in accordance with Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] In the following, specific embodiments of the present inventionwill be explained with reference to the drawings.

[0032] The single-focus lens of the embodiment (representing that ofExample 1) shown in FIG. 1 is one comprising, successively from theobject side, the first lens L₁ having substantially no power with aconcave surface directed onto the object side, a stop 2, the second lensL₂ having a positive refracting power, and the third lens L₃ havingsubstantially no power with a convex surface directed onto the imagesurface in the vicinity of the optical axis, so that a luminous flux isefficiently converged at an imaging position P on a cover glass sheet 1of an imaging device. Here, the second lens L₂ has an imaging function,whereas each of the first lens L₁ and the third lens L₃ is a lens havingan aberration correcting function. Namely, aberrations of the luminousflux incident on this lens system are corrected by the first lens L₁,the luminous flux is converged by the second lens L₂, and then itsaberrations are corrected again by the third lens L₃. Since aberrationsare corrected upstream and downstream the second lens L₂ having animaging function, aberrations can sufficiently be made favorable by acompact, inexpensive, three-lens configuration as well.

[0033] Here, the second lens L₂ is a biconvex lens made of glass with alarge Abbe number having a surface with a stronger curvature on theimage surface side. Each of both surfaces of the first lens L₁ and thethird lens L₃ is an aspherical surface represented by the followingaspheric surface expression, and is configured so as to favorablycorrect various aberrations. A plastic material is used for the firstlens L₁ and third lens L₃ in order that aspheric surfaces can be formedeasily.

[0034] Aspheric surface expression

Z=C·h ²/[1+(1−K·C ² ·h ²)^(1/2) ]+A4˜h ⁴ +A6·h ⁶ +A8·h ⁸ +A10·h ¹⁰

[0035] where

[0036] Z is the length of the perpendicular to a tangential plane (planeperpendicular to the optical axis) of an apex of the aspheric surfacefrom a point on the aspheric surface having a height h from the opticalaxis;

[0037] C (=1/R) is the reciprocal of the paraxial radius of curvature Rof the aspheric surface;

[0038] h is the height from the optical axis;

[0039] K is the eccentricity; and

[0040] A4, A6, A8, and A10 are the fourth-, sixth-, eighth-, andtenth-order aspheric surface coefficients.

[0041] As mentioned above, the first lens L₁ and the third lens L₃ havesubstantially no power and can only function as so-called correctionplates, thus yielding substantially no aberrations. Also, since they arearranged so as to hold the stop 2 therebetween, they can efficientlycorrect various aberrations occurring in the second lens L₂. Since thefirst lens L₁ and the third lens L₃ have substantially no power,aspheric surfaces can be designed easily.

[0042] The first lens L₁ has a concave surface directed onto the objectside, thereby correcting aberrations more favorably.

[0043] The object-side surface of the second lens L₂ has a nearly flatsurface in order to prevent large aberrations from occurring whilekeeping a power as the second lens L₂.

[0044] Though each of both surfaces of the first lens L₁ and the thirdlens L₃ is an aspheric surface, substantially favorable aberrationcorrection can be achieved if at least one surface of each of the firstlens L₁ and the third lens L₃ is provided with an aspheric surface.

[0045] Further, the single-focus lens of this embodiment is set so as tosatisfy the following conditional expressions (1) to (5):

|f′/f ₁<0.5  (1)

0.5<f′/f ₂<2.0  (2)

f′/f ₃<0.5  (3)

1.65>N_(d2)  (4)

50<ν_(d2)  (5)

[0046] where

[0047] f′ is the focal length of the whole lens system;

[0048] f₅′ is the focal length of the first lens;

[0049] f₂′ is the focal length of the second lens;

[0050] f₃′ is the focal length of the third lens;

[0051] N_(d2) is the refractive index of the second lens at d-line; and

[0052] ν_(d2) is the Abbe number of the second lens at d-line.

[0053] Here, the above-mentioned conditional expressions (1) to (3)define respective powers of the individual lenses. The above-mentionedconditional expressions (1) and (3) define powers of the first lens L₁and the third lens L₃ made of a plastic material. In the outside ofthese numerical ranges, environmental changes such as those intemperature and humidity affect optical characteristics more strongly,which is unfavorable. On the other hand, the above-mentioned conditionalexpression (2) defines the power of the second lens L₂ having a strongpower. In the outside of this numerical range, powers caused by plasticlenses of the first lens L₁ and the third lens L₃ increase, which isunfavorable in terms of aberration correction.

[0054] The above-mentioned conditional expressions (4) and (5) definecharacteristics of the glass material for the second lens L₂. In theoutside of these numerical ranges, chromatic aberration is harder tocorrect, which is unfavorable.

EXAMPLES

[0055] In the following, examples will be explained with reference tospecific numerical values.

Example 1

[0056] The single-focus lens in accordance with Example 1 is configuredas explained in the embodiment.

[0057] Table 1 shows values of the radius of curvature R (mm) of eachlens surface, center thickness of each lens and air space between eachpair of neighboring lenses (hereinafter collectively referred to asaxial surface space) D (mm), and refractive index N and Abbe number ν ofeach lens at d-line in this single-focus lens. The surface numbers inthe table successively increase from the object side. The surfaceshaving “*” added to the left side of their surface numbers in Table 1are made aspheric as mentioned above.

[0058] Table 2 shows the respective values of constants K, A4, A6, A8,and A10 of each aspheric surface shown in the above-mentioned asphericsurface expression.

[0059] The focal length f′, Fno, and angle of view 2 ω are set as shownin the lower part of Table 1.

[0060] The single-focus lens of Example 1 is configured so as to satisfyall the conditional expressions (1) to (5) as shown in the lower part ofTable 1.

Example 2

[0061] As shown in FIG. 2, the single-focus lens in accordance withExample 2 has a configuration substantially the same as that of theabove-mentioned Example 1 but differs therefrom in that the object-sidesurface of the second lens L₂ is a concave surface approximating aplane.

[0062] Table 3 shows values of the radius of curvature R (mm) of eachlens surface, axial surface space D (mm) of each lens, and refractiveindex N and Abbe number ν of each lens at d-line in this single-focuslens. The surface numbers in the table successively increase from theobject side. The surfaces having “*” added to the left side of theirsurface numbers in Table 3 are made aspheric as mentioned above.

[0063] Table 4 shows the respective values of constants K, A4, A6, A8,and A10 of each aspheric surface shown in the above-mentioned asphericsurface expression.

[0064] The focal length f′, Fno, and angle of view 2 ω are set as shownin the lower part of Table 3.

[0065] The single-focus lens of Example 2 is configured so as to satisfyall the conditional expressions (1) to (5) as shown in the lower part ofTable 3.

[0066]FIGS. 3A to 3C and 4A to 4C are aberration charts showing variousaberrations (spherical aberration, astigmatism, and distortion) of thesingle-focus lenses in accordance with the above-mentioned embodiments.Each astigmatism chart shows respective aberrations with respect tosagittal (S) and tangential (T) imaging surfaces. As can be seen fromthese aberration charts, the single-focus lens of each of theabove-mentioned examples can favorably correct the aberrations.

[0067] The single-focus lens of the present invention is not limited tothose of the above-mentioned examples. For example, the form of eachlens and the form of aspheric surface can be selected as appropriate.Also, the stop may be disposed between the second and third lenses aswell.

[0068] In the single-focus lens of the present invention, as explainedin detail in the foregoing, a second lens having an imaging function anda stop are held between first and third lenses formed with asphericsurfaces as mentioned above while having substantially no power, wherebyfavorable optical performances can be attained even in a three-lensconfiguration which can meet requirements for compactness and lowercost. TABLE 1 Surface No. R D N_(d) ν_(d) *1 −0.3891 0.278 1.58362 30.2*2 −0.4371 0.432  3 56.8708 0.627 1.49700 81.6  4 −0.5918 0.179 *5−1.8076 0.269 1.58362 30.2 *6 −2.6742 0.695  7 ∞ 0.098 1.51680 64.2  8 ∞

[0069] TABLE 2 K A4 A6 A8 A10 1^(st) surface 0.0831 1.4356 −2.65722.2752 2.6150 × 10 2^(nd) surface −0.1022 0.8857 −1.5730 −1.6110 × 10 1.1455 × 10² 5^(th) surface −2.2030 1.6970 −7.9263  2.4798 × 10 −1.5844× 10  6^(th) surface 0.0945 2.3510 −3.1860 9.3429 2.0551 × 10

[0070] TABLE 3 Surface No. R D N_(d) ν_(d) *1 −0.3802 0.287 1.58362 30.2*2 −0.4294 0.378  3 −36.2435 0.634 1.43875 95.0  4 −0.5230 0.181 *5−1.6085 0.272 1.58362 30.2 *6 −2.2493 0.711  7 ∞ 0.100 1.51680 64.2  8 ∞

[0071] TABLE 4 K A4 A6 A8 A10 1^(st) surface −0.0078 1.3713 −2.56271.5493 2.1276 × 10 2^(nd) surface −0.1836 0.8289 −1.4870 −1.4190 × 10 1.0525 × 10² 5^(th) surface −2.1934 1.5808 −7.6329  2.2960 × 10 −1.0953× 10  6^(th) surface 0.0942 2.2712 −2.8133 9.0543 1.7471 × 10

What is claimed is:
 1. A single-focus lens comprising, successively froman object side, a first lens with a low power having at least oneaspheric surface, a second lens with a positive refracting power havinga convex form on an image surface side, and a third lens with a lowpower having at least one aspheric surface.
 2. A single-focus lensaccording to claim 1, wherein a stop is disposed between said first andsecond lenses.
 3. A single-focus lens according to claim 1, wherein saidfirst and second lenses are formed from a plastic material.
 4. Asingle-focus lens according to claim 1, wherein said single-focus lenssatisfies the following conditional expressions (1) to (3): |f′/f₁′|<0.5  (1) 0.5<f′/f ₂′<2.0  (2) |f′/f ₃′<0.5  (3) where f′ is thefocal length of the whole lens system; f₁′ is the focal length of thefirst lens; f₂′ is the focal length of the second lens; and f₃′ is thefocal length of the third lens.
 5. A single-focus lens according toclaim 1, wherein said first lens has a meniscus form with a concavesurface directed onto the object side.
 6. A single-focus lens accordingto claim 1, wherein said single-focus lens satisfies the followingconditional expressions (4) and (5): 1.65>N_(d2)  (4) 50<ν_(d2)  (5)where N_(d2) is the refractive index of the second lens at d-line; andν_(d2) is the Abbe number of the second lens at d-line.
 7. Asingle-focus lens according to claim 1, wherein said second lens has asubstantially flat surface on the object side.
 8. A single-focus lensaccording to claim 1, wherein each of said first and third lenses has anaspheric surface on each side thereof.